Oscillator stabilizing circuit



Aug. 14, 1945. w. P. B'OLLINGER OSCILLATOR STABILIZING CIRCUIT Filed May 16, 1942 2 Sheets-Sheet 2 7505c. Tues 75050. TUBE 1 V2 INVENTOR WALoo/v RBZL/N ER.

M'UW

ATTORNEY Patented Aug. 14, 1945 UNITED; s-I'Ai'j-E oscn nll'ro t s rannrziiva cmcm'r Waldon {1 'liollinger, I Audubon, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application May 16, 1942, Serial No. 443,193

v12 Claims. (01. 179-1715) This invention relates to improvements in a frequency modulation system, and more particularly to a method of and means for stabilizing the mean frequency of the oscillator in' a frequency modulated transmitter or any transmitter employing a self-excited oscillator controlled by a reactance tube.

Briefly stated, the invention comprises a reactance tube modulator circuit which is so constructed and arranged that the susceptance of the reactance tube varies with oscillator frequency in such direction and magnitude as to restore the oscillator frequency to its assigned mean value, while permitting a controllable change in the frequency of the oscillator by a change in the transconductance of the reactance tube.

In the conventional reactance tube circuit known in the art for frequency modulating a selfexcited oscillator, where the series condenser and shunt arranged resistor of the phase shifting network are both small, the reactance tube acts as a constant capacity whose reactance varies inversely as the oscillator frequency. A common circuit or this type is shown in Fig. 1 which illustrates diagrammatically a conventional reactance tube circuit for frequency modulating a self-excited oscillator. In Fig. 1, the radio frequency oscillator is represented by vacuum tube V: with its parallel tuned circuit L2, Ca connected in a Hartley arrangement. Of course, any other selfexcited oscillator circuit can be used. The reactance tube is represented by Vi, which has its anode coupled to the oscillator through the coupling condenser Ci. The elements of the reactance tube comprise a phase shifting network having a resistance R and a capacity C, both of.

which have small values, and a radio frequency choke coil or so-called decoupling inductance L1- This phas shifting network (in order to give 90 phase shift to the radiofrequency voltage at the grid of tube V1) must of necessity have a small value of R and a small value of C. with this condition then, the RC network of the reactance tube has an amplitude characteristic which causes the-amplitude of the radio frequency grid voltage to vary directly as the frequency of the sp- 1 plied radio frequency anode voltage. As a result of" this, the radio frequency anode current which is the reactive current in the reactance tube cir- R, the reactance of C is large compared with the resistance of R. The source of modulating potentials, usually speech waves. is diagrammatically represented by the label IN. A battery B serves to supply suitable bias to the grid of the reactance tube.

According to one embodiment of the present invention, the reactance tube circuit is so constructed and arranged that its capacity is made to vary as the .square of the frequency or the reactance vary as the one-third power of the frequency. For this condition, the adio frequency grid voltage applied to the reac nce tube varies as the cube of the frequency I with it as heretofore. Thus, y making the-capacity of the reactance tube ary with theapplied radio frequency instead 0 being constant, I am able to make this varia n in such direction and magnitude as to subs tially prevent the selfexcited oscillator fr departing from a given frequency. To ac eve this result, I cause a change in freque cy of the oscillator to result in a change of ca city of the reactance tube, thus restoring the oscillator to its original operating frequency. This stabilization of the oscillator frequency prevents such factors as changes in 'grid or any other suitable control element of the reactancetube.

A more detailed description of the invention follows in conjunction with a drawing, wherein Fig. 1 illustrates a conventional circuit of a reactance tube and self-excited oscillator whereby the frequency of the oscillator can be modulated,

and Figs. 2, 3, 4, 5 and 6 show different embodicuit, will vary directly as the frequency of the radio frequency voltage applied to the anode.

tube V1 having three C, R circuits for enabling capacity whose reactance varies inversely as the:

frequency. Because of the small values of C and ments of the present invention. In the drawings, the same parts are represented by the same reference numerals. Figs. 2 to 6, inclusive, merely illustrate those parts which are necessary for an understanding of the principles of the present invention, it being understood that the circuits of thesefigures can be coupled to the radio frequency oscillator in the same manner shown, in

Fig. 1.

Referring to Fig. 2, there is shown a reactance the capacity of the reactance tube to vary as the as the cne-thi'rd powerof the frequency. Thisis'equiva'lent to. making the grid radio frequency voltage vary. as; the cube (third power. of the" oscillator frequency The three C,- R circuits arerepresentedby- C', R; C R; C", R".. Thethree capacities-C, C; C aresmall. and ofthe.

same value, and similarly the three resistors R,

R R" are also small'and of the same value.

Since each. of the-two'additional-RC circuits (overv and above: the' singlezone shown in Fig. 1) produces a 90 phase shift, it will-.be obvious that thereisan additional 180 phase shift applied to theireactance tube-over and a'bove. the phase shift obtained from a single R, C. circuit. In other '1 words. the network produces a phase shift of 270 In. order. tov correct. forthis additional 180 pliasershiftso as to producethe desired'quadrature or QO 'phase relation between the platevoltage and the grid. voltage, the coupling coil L: (which abstracts highfrequencyenergyfrom the radio frequency oscillator tank) is. suitably poled;

In eifeet,-the;tliree: capacity and: resistance net.-

' works of-"the reactance -tubeof Fig.. 2.constitutei a high pass filter circuit, butthesyst'emis. so designedv that. the reactance tube'operates on. thelinear: slope portionof: the high pass filter. char.-

acteristic of. frequency versus'grid voltage. The circuitof Fig. 2includes the usual grid leak condenser combination herein represented as Cg, Rg, but these have such'valuesas: not to alter'the desired phase shifting. v characteristic of" the" network. The. modulating; source oflspeech waves is diagrammaticallyrepresented by m while a bat tery'B isabias'battery.

Inthe' operation of Fig. 2, a' decreasev in the oscillator frequency-will produce a great loss in theradio frequency voltage on the gridof there'- actance tube, and: the reactance tube will'act like azsmaller'eapacity, thereby tending to increase the oscillator frequency. Conversely, a rise intheoscillatorfrequency will produce a great increase in: the. radio. frequency voltage. onthe grid ofth'e reactance tubeandthis reactance tube will act asia; larger. capacity thereby tending to lower theoscillating frequency. V

Fig; li illustrates another modification of the invention. and is an. alternative arrangement to the" system: of Fig. 2. Instead of the inductive coupling: between thev oscillator and the grid. of the reactance tube, as shown: in Fig. 2, there. is here provided a direct coupling (except forv the phase shifting networks) between the anode and grid ofithereactancetube. The circuit of Fig. 3 includes, itishouldbe noted, two additional R,.C

. circuits. rpresented'byR C' and'R"",.C"

which secure the additional 180 phase shift'for' BOT In Fig; 4, the. objects: of the invention. are

achieved. by working on the effective inductance of the reactance tube instead of. on1the effective capacity thereof. is' doneby using, capacities ofthe network .inshunt and the resistorsin series, as shown. The threen'etworks of Fig. 4 are ,R's, C3; R4, Cq and B5, C5. The resistorsRa, R4,

Rs are large and of. thesame value, and the condensers C3, C4, C8 are also large and of the same asaaroe value; It should be understood; of. course, that the valuesof the grid leak. and condenser combination-are such as not to upsetthephaseshifting characteristic of the network. Thexvol-tage appliedto the grid is derivedfrom the oscillator V tank circuit'by virtue' of 'the small coupling coil L3, in substantially the same manner. asxheretofore described in connection with Fig. 2; In'Fig. 4, in effect, the resistance and capacity. constitute. a low pass filter circuit, but here again we operate: on the. linear. slope portion of. the phase-shifting network. characteristic.

In Fig. 5; which. showsanother embodiment of. the-invention, we obtain aneilective capacity for thereactance tube by using inductors; La, D1 and La in shunt, and resistors R6, R7 and Ra inseries for the phase shifting network. Theresistances R6, R1 and Rs are of the same value and relatively large and. the inductances L5, L7 and Lnare equal but of small value. Here again, it. is

desired that the rea'ctance tube work on-the linear: portion of the phase shif-ting network character..-

istic.

Fig. Sis-still another embodiment of theinventionemploying inductances L10, Lu, and Drain series, and resistors Rm, Rnand R12 in shunt. The;

coils L10, Lu and L1: are relatively large and'equal in value, while the resistors-Rm, Ru and R12 are relatively small in value and equal. The, network of Fig. 6, in effect, is a low pass filter. upon whose linear slope the circuit works.

It' is understood that the resistances in the: phase shifting network. do not have to be equal for proper operation of the circuit but for conven-- ience of discussion and construction are. saidto' be equal. The sameapplies to the capacities o "inductances.

v *=.The same theory of operation hereinabove-described in connection with Figs. 2 and 3 applies generally to the systemof Figs. 4; 5 and 6, except that in Figs. 2, 3 and 5 the reactance'tube functions as an eflective capacity, while inFigs. 4 and 6 the reactance tubefunctions as inductance.

a H What isclaimed-is:

1. A stabilized oscillation generator comprising "a vacuum tube oscillator having a frequency determining circuit, a reactance tube havinga control electrode and a cathode and coupled to saidv oscillator, a phase shifting network coupled between said control electrode and the cathode of said reactance. tube, and a: connection coupling said network to said frequency determining circuit, said network having a plurality of lumped reactance elements of the same kind and a plurality offresistor elements, the number of said,

elements and their values being so related as to produce a phase shift substantially equal to an odd multiple of. but notsubstantially less than 270, said elements being'so'arranged that thesusceptance of said reactance tube varies with the frequency of theoscillations producediby said oscillator in such direction and magnitude. as to.

oppose change's in the oscillator frequency from its assigned value. s

2.. A stabilized oscillation generator comprising",

a vacuum tube oscillator having a frequency do.

'termining circuit, a reactancetube coupled to said oscillator, and a phase shifting network coupled between a: control element and the: cathode. of said reactance tube, and a connection. coupling.

said network to said frequency determining circuit, said network having lumped reactance and resistor'elemen-ts, the number of said elements and their values being so related as to. producean effective.

a phase shift substantially equal to an odd multiple of 90' but not substantially iessxthan 2'10", said elements being so constructed and arranged that the effective capacity of said reactance .tube' varies with the frequency of the oscillation producedby saidoscillatorin such direction and mag- ,assaios nitude as to oppose changes in the oscillator fre- 1 quency from its assigned value.

3. A stabilized oscillation generator comprising a vacuum tube oscillator having a frequency determining circuit, a reactance tube coupled to said oscillator, and a phase shifting network coupled between a control element and the cathode of said reactance tube, and a connection coupling said network to said frequency determining circuit, said network having lumped reactance and resistor elements of .such number and of such related values that there is produced a' phase shift substantially equal to an odd multiple of 90 but not substantially less 'than.270, said elements being so constructed and arranged'that the etfece tive inductance of said reactance tube varies with the frequency of the oscillations produced by said oscillator in such direction and magnitude-as to oppose changes in the oscillator frequency from its assigned value.

4. A stabilized oscillation generator comprising a vacuum tube oscillator having a frequency determining circuit, a reactance tube coupled to said oscillator, and a phase shifting network of series capacitance elements and shunt resistance elements coupled to a control element of said reactance tube and also to said frequency determining circuit, theconstants of said network being so chosen that it operates on the substantially linear slope portion of its filter characteristic, the number of said elements of said network and their values being so related as to produce a phase shift-substantially equal to an odd multiple of but not substantially less than 270, whereby the eflective capacity of said reactance tube varies with the frequency of the oscillations produced by said oscillator in such direction and magnitude as to oppose changes in the oscillator frequency from its assigned value.

5. A'stabilized oscillation generator comprising a vacuum tube oscillator having a frequency determining circuit, a reactance tube coupled to said oscillator, and a phase shifting network of series resistance elements and shunt capacitance elements coupled to a control electrode of said reactance tube and also to said frequency determining circuit, the constants of said network being sochosen that it operates on thesubstantially linear slope portion of its filter characteristic, the number of said series and shunt elements and their values being so related as to produce a phase shift equal to an odd multiple of 90 v but not substantially less than 270, said capacitor elements and resistor elements being so constructed and arranged that the effective inductance of said reactance tube varies with the frequency of the oscillations produced by said oscillator in such ments of the same value and shunt impedance elements of the same value, the constants of said elements being such thatthe network operates on the linear slope portion of the filter characteristic.'the number of said elementsand their values being so related as to produce a phase shift substantially equal to an odd multiple of 90 and not substantially less than 270, the elements being so constructed and arranged that the susceptance of said reactance tube varies with the frequency of the oscillations produced by said oscillator in such direction and magnitude as to oppose changes in the oscillator frequency from its assigned value.

7. In a frequency modulation system, a radio frequency oscillator, a reactance tube having a phase shifting network coupled to said oscillator, a low frequency modulating source coupled to.a

control element of said reactance tube for proof the same kind, while the elements of the other plurality are resistors, the number of. said series and shunt elements and their values being so related as toproduce a phase shift substantially equal to an odd multiple. of 90 but not substantially less than 270, said series and shunt elements being so constructed and arranged that the susceptance of said reactance tube varies with the frequency of said oscillator in such direction and magnitude as to oppose changes in the oscile lator frequency from its assigned value, while permitting controllable changes in the-oscillator frequency by changes in the transconductance of the reactance tube.

-8. A stabilized generator comprising a vacuum tube oscillator having a frequency determining circuit, a reactance tube coupled to said oscillator, and a phase shifting network having a high pass filter characteristic coupled to a control element of said-reactance tube and also coupled to. said frequency determining circuit, said network having aplurality of capacitor elements of small value in series and a plurality of resistor elements of small value in shunt, the constants of said network elements being such that the network operates on the linear slope portion of the filter characteristic, the number of said series and shunt elements and their values being so related as to produce a phase shift equal to an odd multiple of.

90 but not substantially less than 270, said capacitor elements and resistor elements being so constructed and arranged that the susceptance of said reactance tube varies with the frequency of the oscillations produced by said oscillator in such direction and magnitude as to oppose changes in the oscillator frequency from its assigned value.

9. A stabilized generator comprising a vacuum tube oscillator having a frequency determining circuit, a reactance tube coupled to said oscillator, and a'phase shifting network having a' low pass filter characteristic coupled to a control element of said reactance tube and also coupled to said frequency determining circuit, said network having a plurality of resistor elementsof large value in series and a plurality of capacitorrelements of large value in shunt, the constants of said network elements being such that the network operates on the linear slope portion of the filter characteristic, the number of said series and shunt elements .and'their values being so related as to produce a phase shift equal to an odd multiple of 90' but not substantially less than 270",

"said capacitor elements and resistor elements betively coupling said network and said frequency determining circuit, a source of modulatingpurrents coupled to a control element of said re actance tube for producing controllable changes in the frequency of said oscillator in accordance with changes in the transconductance of said reactance tube, said network having a plurality'of series elements and a plurality of shunt elements of such constants that the network operates on thesubstantially linear slope portion of its filter characteristic, the elements of one of said plu-.

rality being reactances of the same kind, while the elements of the other plurality are resistors, the number of said elements and their values being so related as to produce a phase shift substantially equal to an odd multiple of 90 but not substantially less than 270, said elements being so constructed and arranged that the susceptance of said reactance tube varies with the frequency of said oscillator in such direction and magnitude as to oppose changes in the oscillator frequency from its assigned value, while permitting controllable changes in the oscillator frequency by changes in the transconductance of the reactance tube.

11. In a frequency modulation system, a radio frequency oscillator having a frequency determining circuit, a reactance tube, a phase shifting network coupled between a control electrode and the cathode of said reactance tube, a capacitive connection between said network and said frequency determining circuit, a source of modulating currents coupled to a control element of said 4 means reactance tube for producing controllable changes in the frequency of said oscillator in accordance with changes in the transconductance of said reactance tube, said network. having a plurality of 6 series elements and a plurality of shun-t elements of such constants that the network operates on the substantially linear slope portion of its filter characteristic, the elements of one of said plurality being reactances of the same kind, while 10 the elements of the other plurality are reactors,

the number of said series and shunt elements and their values being so related as to produce a phase shift substantially equal to an odd multiple of 90v but not substantially less than 270, said series .15 and shunt elements being so constructed and arranged that the susceptance of said reactance tube varies with the frequency of said oscillator,

insuch direction and magnitude as to oppose changes in theoscillator frequency from its assigned value, while permitting controllable changes in the oscillator frequency by changes in the transconductance of the reactance tube.

12. In a frequency modulation system, a'radio frequency oscillator, a reactance tube having a phase shifting network coupled to said oscillator,

a low frequency modulating source coupled to a control element of said reactance tube for producing controllable changes in the frequency of said oscillator in accordance with changes inthe transconductance of said reactance tube, said network having a plurality of series elements and a plurality of shunt elements of such constants that the network operates on the substantially linear slope portion of its filter characteristic, the numher of said series and shunt elements and their values being so related as to produce a phase shift substantially equal to an odd multiple of'90 but not substantially less than 270, said series and shunt elements being so constructed and arranged 40 that the susceptance of said reactancectube varies with the frequency of said oscillator in such direction and magnitude as to oppose changes in the oscillator frequency from its assigned value, while permitting controllable changes in the oscillator frequency by changes in the transconductance of the reactance tube.

WALDON P. BOLLINGERZJ' 

